# Suddenly, you live in the dark!

Imagine that you lived in this world:

• The planet has a diameter of 5000 km.
• The rotation time is the same as the orbital period.
• And here it comes: the orbital period is 1027 years.
• The planet's orbit is similar to Pluto's, only much more elliptical.
• For just over 30 days, the planet is in the inner solar system, and then runs away to the distant darkness for 1026 years.

I wonder if this would be possible, and whether the planet could get so far from the sun without escaping it entirely?

And here comes the predictable question: Could there be life on such a planet?

I don't ask life like on Earth, of course: It would be adapted to darkness, and able to survive in very low temperatures. Still:

• In outer space, over so many centuries, the temperature gets really low.
• Could a species accustomed to the darkness possibly adapt to the light and heat of the sun for a few days?
• Could they have somehow developed a technology that "captured" the heat from the brief time near the sun, and would store and release it over 1026 years?

I know, it probably sounds very unrealistic, but I'd really like to see if something like this could be plausible.

• Check out A Deepness in the Sky by Verner Vinge. Their sun turned off for 200 years and was on for a 30 years, or something like that. – JDługosz Dec 27 '14 at 1:43
• The planet reaches 5000 km : is this the radius ? – Vincent Dec 27 '14 at 2:07
• Just a tangential note, but there's no reason to expect a planet with such a hugely eccentric orbit to have synchronous rotation. Even if its rotation did become resonantly locked, it would almost surely fall into a higher resonance than 1:1. Even Mercury's orbit is eccentric enough to keep it in a 3:2 resonance, and its orbit is nearly circular compared to you planet's. – Ilmari Karonen Dec 27 '14 at 13:14
• Similar idea to Nightfall, but different. – Shokhet Dec 28 '14 at 19:49
• There's another issue with the tidal locking of an eccentric orbit (beyond the fact that it's not likely to occur). Unlike a regular tidal locking, the star wouldn't actually appear stationary in the sky. This is because the rotational velocity would be constant, but the orbital velocity would not be. As seen from the "prime meridian", the sun would start directly overhead from its furthest point, then slowly approach one horizon, then get larger as the planet gets closer, then swiftly pass the zenith to the opposite horizon, then retreat smaller again, then slowly go back to center. – Caleb Hines Dec 28 '14 at 23:19

## 7 Answers

Life growing on such an unusual planet would certainly reflect its orbit. There would be virtually no movement during the cold period. During the summer "month," life would have to reap all of the energy it can, and then lie dormant again.

There are shorter term examples of this extreme seasonal behavior on Earth. There are plants growing in the Australian Outback which leave their seeds dormant for years, waiting for the rains. They don't respond to the first rain in a year, they respond to the second. This indicates an unusually "wet" year and signals a good time to grow.

During the summer, the flora would be extravagant, using all of the energy they can to further their existence.

Interesting things could form near cracks if you have a warm core, but I'm guessing you're interested in a cold core.

I would not be surprised if you saw life find an alternative method of evolution besides simple sexual and asexual reproduction. The long periods in the cold would be rough and creatures waking up would need to "inventory" their genetic code for mistakes over the years. Creatures which can identify and spread genetic material faster stand a better chance of survival.

I would also expect species which defy categorization along Earth lines. In the cold, spores would fare the best, an isolated multitude of genetic copies. As the spring arrives, plant-like life would flourish, getting a head start on the rest of the ecosystem. As summer arrives, animal life would flourish (especially short lived creations like Mayflies). In the fall, the plants would gather energy for one last hurrah, showering spores across the planet.

Given how strong such evolutionary forces would be, I would not be surprised to see species which rely on all three forms of life: ten centuries waiting as spores, just to wake up in the spring and attempt to gather all of the energy they can. In the summer, they would go under a transformation to get a head-start on evolution: they would spawn a multitude of Mayfly like animals whose entire purpose in life is to "discuss" the genetic viability of the plants that spawned them and identify what the next "crop" will contain genetically. They would then "polinate" their host plants and ceremoniously die by walking into a honeypot trap to be turned into the raw materials needed to spew billions of spores across the sky. The plants would then die off as the next winter begins, and the planet would lie in wait.

Of course this is just one exotic answer. The joy of inventing a planet like your is that you have limitless options as to how things can evolve. Your imagination is the limit.

The orbit you describe is typical for (moderately) long-period comets.

Given the desired orbital period, we can calculate the semi-major axis of the orbit (which, for a highly elliptical orbit like this, is approximately half its maximum distance from the star) using the formula $$T^2 = \frac{4\pi^2}{GM}a^3,$$ where $T$ is the orbital period, $a$ is the semi-major axis, $M$ is the mass of the star (the mass of the planet is assumed to be negligible in comparison) and $G$ is the universal gravitational constant. However, if we assume that the planet is orbiting a star of about the same mass as our Sun, and if we measure time in years and distance in AUs, we can take a shortcut: we know the Earth orbits the Sun in one year, with a semi-major axis of one AU, so we must have $a=1$ when $T=1$. The constant of proportionality thus simply becomes $1$, and we're left with $a = T^{2/3}$. For $T=1027$ (years), this works out to $a\approx 101.8$ (AU), so our hypothetical planet gets no further than $2a\approx203.6$ AU from the Sun.

This is beyond the classical Kuiper belt, but much closer to the Sun than the Oort cloud, which is believed to be the source of most long-period comets. It is also closer to the Sun than several recently discovered "detached objects" such as 90377 Sedna, which are believed to have relatively stable orbits. Your planet thus remains well within the Sun's gravitational well, and is at little risk of being scattered by passing stars. (Well, at least as long as they don't pass too close, in which case we could all be in trouble anyway.)

There is a problem, though.

Your planet might be safe and stable as long as it's far away from the Sun, but as soon as its orbit takes it into the inner solar system, it's going to occasionally pass close to to the planets already there, and thus be affected by their gravity. Such interactions could have many possible outcomes, but the statistically most likely one (at least in a solar system similar to ours) is a close pass with a large gas planet like Jupiter, which will either fling the "comet planet" straight out of the system, or, more likely, kick it into a shorter-period orbit that will keep it interacting with the other planets in the system until it gets ejected from the solar system, collides with the Sun or one of the planets, or (least likely) get captured into a stable near-circular orbit.

So, to keep our hypothetical planet safe, we really should clear the solar system of any inner planets that might disturb its orbit. Fortunately, that might not be so implausible — something obviously caused our planet to end up in that highly elliptical orbit in the first place, and it's not altogether implausible that the same something (which could've been, say, another star passing through the inner solar system) might have also conveniently swept the system clean of any inner planets in more conventional orbits. (Indeed, our planet might well have been one of those, before it was scattered into its new eccentric orbit.)

OK, so we've got the orbit stabilized. What about the climate?

First of all, let me note that there's absolutely no reason to expect a planet with an orbit like this to be tidally locked, and certainly not to a 1:1 resonance like you suggest. On such an eccentric orbit, the planet would feel almost no tides at all for most of the time, and would only experience a single tidal nudge every time it passed close to the Sun. If anything, if it locked at all, it should therefore lock to a rotational speed approximately matching its orbital angular velocity near perihelion. But I suspect it just wouldn't lock at all, but would simply retain whatever rotational period it originally started with.

So what's the weather like? Well, for most of its orbit, this planet is going to be so far from the Sun that it might as well be in interstellar space — the Sun will just be the brightest star among many in the sky. Without any significant heat from the Sun, any seas the planet may have will all but freeze solid, and even the atmosphere will freeze and snow down. In general, the surface of such a planet seems pretty inhospitable for life, or at least any kind of life as we know it.

That said, if the planet is large enough, and possesses a massive rocky core, it's possible that the bottom layers of the oceans might stay liquid due to geothermal heat (from trapped primordial heat, and from decay of radioactive elements in the core). Such a subglacial ocean could potentially support life, even if the available free energy would likely be orders of magnitude less than that on Earth's surface.

Thus, we might expect any life on such a planet to be aquatic, and likely to cluster around hydrothermal vents at the bottom of the ocean, like some deep-sea ecosystems do on Earth. Generally, I would not expect life under such conditions to evolve very fast, but in science fiction, even unlikely events are still possible, and so of you want to postulate advanced, even intelligent life to have evolved under such conditions, I'd be happy to suspend my disbelief.

To such an ecosystem, living under a kilometers-thick blanket of ice, safe from to freezing cold of space, would be the norm. The rare and brief passes close to the Sun, if felt at all beneath the thick ice blanket, would be abnormal and catastrophic events: suddenly, scorching rays of heat blast down from what's normally the coldest part of the world, melting the ice and upsetting the ocean circulation.

Of course, any life that did survive such events would eventually adapt to them, but they couldn't possibly be easy on any hypothetical subglacial civilization on your world. Then again, that could make for an excellent story...

• You know, +1 for having the guts to drag the relevant math into it, and for an answer after my own heart. Excellent climate analysis; do you think that there would be any tidal forces at work on the planet, as there is between Jupiter and some of its moons (I'm guessing nothing of any importance because of the enormous distances we're talking about here). – HDE 226868 Dec 28 '14 at 0:18
• @HDE226868: Most of the time, I'd say no, unless the planet happens to have a massive moon of its own (and I have no idea how stable that could be, with such an eccentric orbit) and isn't tide-locked to it (like Pluto and Charon are). There would be solar tides at perihelion, which could well make things even more interesting -- not only is the solid ice crust suddely melting, but it's flexing and buckling too! Depending on stuff like the rotation rate, the perihelion distance and the thickness of the ice, it might seem like the mother of all earthquakes. – Ilmari Karonen Dec 28 '14 at 0:24
• One way to reduce the chance of being ejected would be to give it an orbit inclined well above the plane of the ecliptic. Eris, for example, is 44° above. – Schwern Dec 29 '14 at 7:16
• The idea of a relatively normal star system that gets disrupted might also explain more complex life. If life originally evolved in a more normal orbit and then had to adapt to the radically changed orbit that might work. – Tim B Dec 29 '14 at 9:57

There would be a big problem with life on such a world. As Cort Ammon says it's quite possible for life to survive in such an environment. Evolving is another matter, though. Single cell life will survive it as it generally can take being frozen. Multi-cellular life is another matter--I don't think there's anything that can survive that kind of cold other than in a totally dormant state (seeds, spores etc.) and multi-cellular life isn't going to start out that way. Thus you have an uncrossable evolutionary chasm.

The only way I see to have non-microbial life on such a world is if it started out as a terrestrial world and it's orbit slowly got stretched--enough time for plants (and conceivably even egg-laying animals) to adapt.

Note, also, that such an orbit (and it's months of sun, not days) means life spends almost all it's time in the dormant state, it can only evolve during the warm periods. It's going to evolve VERY slowly. While it has billions of years of orbiting it's star it will only evolve like it's been millions--you will not see any major changes from whatever life looked like before the orbit got stretched.

• Exactly. Life surviving somehow: possible, very low life forms. Evolving? Very unlikely. – Peter M. Dec 29 '14 at 15:53

Could there be life on such a planet? Yes, of course. Something worth researching, if you're ever looking for inspiration, are Extremophiles and Tardigrades.

Extremophiles are species which are adapted to extreme environments, whereas Tardigrades are capable of surviving in extreme environments, but are not adapted to them.

There are a lot of options here; you could have a species which freezes itself for the 1026 years, only to reanimate shortly before the sun cycle. There are many species of insects which appear only shortly (a few days) during a season to breed and reproduce; you could have something similar where they use the coldness of space to hibernate or suspend animation until the warmth comes back around.

On the flip side, what if the species are adapted to the cold, surviving off of minerals on rocks and good ol' natural selection, but are unable to withstand the 30 days of warmth? Every 1026 years they would either have to burrow closer to the cold core (assuming a cold core) to hide from the heat / UV of the sun.

>> Could they have somehow developed a technology that "captured" the heat from the brief time near the sun, and would store and release it over 1026 years?

The first thing this makes me think about is how the warmth of the 30 days would react with a normally cold planet.... Let's say the surface is dry; only rocks and minerals. But let's say there's water beneath the surface, but frozen (obviously). During 30 days of warmth, depending on how sudden the warmth comes on, it could cause cracking / shifting on the surface, releasing springs of water or pressure from beneath the surface to crack open.

And that's essentially how we generate electricity (take coal, pound it into talcum powder, ignite it in an enclosed space, filled with water pipes. When the water turns to steam, the pressure triggers a valve which pushes the steam through a turbine, inside of which is a magnet and copper wires - hence, electricity). A planet with plenty of minerals (copper, magnets, chemicals, etc), but lacking water, could use released pressure from inside the surface to "capture" the energy as electricity, storing it in fuel cells.

If electricity generation is a bit straight-forward; if you're looking for something more "magical", then what about capturing light? Definitely a bit beyond what's currently considered "plausible", but perhaps they found a way to trap the visible light (or UV) from the 30 days in containers (think an orb which is a one-way mirror; after the light gets in it cannot get out). As such, they could "release" the energy / light over 1026 years, but it would be a treasured resource.

In any case, I don't think you're outside the realm of what's plausible. Half the fun in making planets is making things beyond what we would normally think.

Note: I used the software Universe sandbox2 to get the numbers used in my answer.

Suopposing a planet about the size of Earth having an orbital period of more or less 1027 years around a sunlike star, it would be at 15 billion kilometres or 102 AU from the star. That is very far. You are still considered in the star system but it's too cold to have liquid water. In this test I used an orbit that was almost a perfect circle, but it gives a good idea of the situation. With an ellipse, the planet would be further away. How close and how far can it go, I don't know for sure but the coldest is too cold for life as we know it.

Other form of life could sustain the cold temperatures but if there is a large temperature difference between the hottest and coldest points, life might not be possible.

More about exotic form of life: Is it possible for life to evolve on planets without water?

• The orbital period depends on the semi-major axis, which equals the orbital radius for a circular orbit, and approximately half the maximum distance for a very highly elliptical orbit. So a highly elliptic orbit with a period of 1027 years around the sun would get about 204 AU from the sun. – Ilmari Karonen Dec 27 '14 at 22:34

Look at the orbit of comets. You can plot the exact path and timing. You'll find it spends several months in the vicinity of the sun. You're nowhere near losing the sun's grip: look up Sedna.

I think the planet's surface will always become ridiculously cold, but if the planet has a molten core, that might allow some warm enough underground areas. If the crust has an extensive cave system that can remain stable enough for life to move in and out as needed, that might present a possibility.

A different planet with a very long day cycle but more consistent heat in the lighted region, might be able to support animals which can migrate around the planet, staying in the light.

If you are looking for different values for your orbit, consider that the star's size and brightness can also vary.